In general, an electric storage element of an electric storage apparatus includes electrode plates stacked in a layered manner, in order to improve electricity storage efficiency. As the wiring configuration of this electric storage element, metal foils extending outward from respective electrode plates are bundled and joined directly or indirectly to wiring members in many cases.
In this case, ultrasonic welding technology is widely used as the above-described technique of joining metal foils.
The above-mentioned metal foils are ultrasonic-welded not simply by causing ultrasonic vibration to act upon the metal foils. Specifically, the method of ultrasonic welding is as follows. As described in Patent Literature 1 cited below, a metal member (first metal member) is sandwiched between a tool (the leading end of a so-called horn) used to cause ultrasonic vibration to act upon the metal foil and the metal foil, and a metal member (second metal member) is also disposed on the opposite surface of the bundled portions of the metal foil. As described above, the bundled portions of the metal foil are sandwiched between the first metal member and the second metal member and, under that condition, ultrasonic vibration is caused to act upon the first metal member to join the metal foil to the metal members, thereby protecting the metal foil.
In addition, ultrasonic welding is performed in a plurality of places to secure a junction area, while avoiding an increase in the size of the leading end portion of the horn.
In the above-described conventional configuration, however, the metal member (first metal member) used to protect the metal foil may become damaged in some cases, as the result of ultrasonic welding being performed in a plurality of places.
That is, when ultrasonic vibration is caused to act upon the first metal member, the first metal member deforms in an undulating manner in the vicinity of each position for ultrasonic vibration to act upon. Thus, the above-mentioned deformations arising from both sides of adjacent positions become superimposed on each other between the two sites of action of ultrasonic vibration.
This superimposition of deformations on each other may in some cases result in such a degree of deformation as to produce cracks ascribable to the stress of the first metal member. If such cracks arise, the relative positional relationship between the first metal member and the metal foil shifts from a preferred positional relationship, thus leading to, for example, yield decline. Consequently, productivity may degrade in some cases.